The present invention refers to a cartridge adapted to contain a single serving of a particulate substance, extractable by means of water for preparing a beverage, preferably an espresso coffee beverage, comprising an essentially cup- or bucket-shaped main body portion having a bottom portion and an open end opposite to the bottom portion, and a cover member adapted to be sealingly attached to the open end of the main body portion.
Such cartridges are well known in the art in a variety of different embodiments, whereby particularly cartridges containing a single serving of coffee powder for preparing an espresso coffee are in widespread use. One of the fundamental advantages of such cartridges may be seen in the fact that they are gas tight, thus keeping the ground coffee contained therein fresh over an extended period of time. For brewing the coffee powder received in the cartridge, predominantly semi-automatic espresso coffee machines are in use whereby the cartridge is inserted into a cartridge holder of the machine, with its cover facing downwards. Then, the cartridge holder is inserted manually into the coffee machine. The coffee machine is provided, in the region of the area where the cartridge holder is to be inserted into the machine, with a so-called brewing spike, i.e. a hollow piercing member comprising radial outlet openings for feeding the brewing water into the interior of the cartridge, once the bottom of the cartridge has been pierced upon insertion of the cartridge holder into the coffee machine. The bottom of the cartridge holder itself is provided with a plurality of projections located on a collecting tray. These projections penetrate the cover of the cartridge and provide a plurality of openings in the cover as soon as pressurized brewing water is fed into the cartridge and the cover thereof is pressed against the projections due to the hydraulic overpressure in the interior of the cartridge. During the subsequent brewing operation, the brewing water is fed into the interior of the cartridge through the brewing spike, with the result that it flows under pressure through the coffee powder in the cartridge, finally leaving the cartridge as coffee beverage through the openings in the cover. The freshly brewed coffee is collected in the collecting tray and flows through a beverage outlet out of the machine. A coffee machine comprising such a setup for the extraction of the coffee powder contained in a cartridge is disclosed, for instance, in the document EP 0,512,470.
The disadvantage of a brewing spike having radially extending outlet openings may be seen in the fact that the cross sectional area of the brewing spike relevant for the outlet of the brewing water relatively quickly clogs under the influence of contaminations. As a reason therefore, for instance fat contained in the coffee powder, coffee powder particles, lime in the brewing water etc. can be named. However, the brewing spike being gradually more and more contaminated, the quality of the brewed coffee beverage gradually decreases. As far as the hydrodynamic standpoint is concerned, the problem in using brewing spikes of the kind referred to herein before is the fact that so-called dead volumes exist in the interior of the cartridge, with the result that portions of the coffee powder are extracted not at all or only to a minor extent.